1. Field of the Invention
The present invention relates to a wireless communication system. More particularly, the present invention relates to a method and an apparatus for supporting a handover in a heterogeneous network.
2. Description of the Related Art
Recently, as demand for data communication and various communication services in a wireless communication system have increased, various alternatives for increasing a transmission capacity have been suggested. One alternative that is being developed is a technology for a heterogeneous network. The heterogeneous network denotes a communication system where base stations having various phases, cell coverage, and characteristics are mixed and operated.
FIG. 1 illustrates a heterogeneous network system according to the related art.
Referring to FIG. 1, the heterogeneous network denotes a system where a macro cell 100 having and one or more pico cells 102, 104, and 106 overlay and operate. In addition, though not illustrated in FIG. 1, the heterogeneous network may further include at least one femto cell inside the macro cell 100.
In the heterogeneous network, the small cells (i.e., the pico cells 102, 104, and 106 or the femto cell (not illustrated)) existing inside the macro cell 100 may reuse a transmission resource to increase a transmission capacity of an entire system. For example, in a case where one small cell having a transmission band of 10 MHz is installed based on the assumption that the macro cell 100 can utilize a transmission band of 10 MHz, a transmission band of 20 MHz in total may be secured. That is, in a case where N small cells are installed in one macro cell, a transmission band corresponding to N times an existing transmission band may be secured, and it may be expected that a transmission rate thus increases N times.
However, in an actual environment, even when N small cells are installed inside one macro cell, an N-times transmission rate cannot be obtained but a smaller transmission rate may be obtained due to interference between heterogeneous cells or a non-uniform distribution of terminals. That is, in an actual heterogeneous network system, performance deterioration may occur due to interference between a macro cell and a small cell, or interference between small cells. In addition, due to non-uniform distribution of terminals with respect to small cells, an unutilized resource may occur and thus performance deterioration may occur.
FIG. 2 illustrates a distribution of terminals in a heterogeneous network system according to the related art.
Referring to FIG. 2, in the heterogeneous network where a plurality of pico cells 211 to 213 exist inside a cell 202 of a macro base station 200, each of a plurality of terminals 220 to 224 measures signal strengths of respective cells to select a serving cell. Accordingly, as illustrated, a pico cell A 210 and a pico cell C 212 may allocate a resource to respective terminals 221 and 225 which have selected the pico cell A 210 and the pico cell C 212 as serving cells. However, a pico cell B 211 and a pico cell D 213 do not have a terminal that has selected the pico cell B 211 or the pico cell D 213 as a serving cell, so the pico cell B 211 and the pico cell D 213 cannot allocate a resource. Therefore, a transmittable resource amount increases due to the pico cell B 211 and the pico cell D 213, but improvement of an actual transmission rate is not achieved. That is, in a case where the number of pico cells that cannot allocate a resource to a terminal is large inside the heterogeneous network, transmission efficiency of the heterogeneous network is reduced.
Therefore, to overcome this disadvantage, recently, a range expansion technology for arbitrarily expanding a service area of a small base station having a narrow service area in the heterogeneous network to provide a service to a greater number of users has been developed. In more detail, in a case where a cell that should service a large number of terminals and a cell that services only an extremely small number of terminals are adjacent to each other, the range expansion technology gives a biased value to the cell that services only an extremely small number of terminals and allows the cell to be selected, so that all cells have an average service terminal distribution.
However, in a case where a relevant terminal selects a pico cell as a serving cell despite an excessively poor channel quality due to the range expansion technology, the relevant terminal may not receive a control signal or an essential broadcast channel due to low channel quality.
Therefore, a need exists for a method and an apparatus for efficiently supporting a handover in a heterogeneous network where a plurality of small cells overlap inside a macro cell.